Most liquid crystal (LC) devices are made from a sandwich of liquid crystal between two spaced glass substrates coated with a transparent conductor. The glass substrates are generally held together to a predetermined gap using an epoxy-based gasket (edge seal) at the edge and are referred to as a panel. The liquid crystal is injected into the gap of the panel using either a vacuum filling process or one drop filling process. In the case of vacuum filling process, the gasket around the panel is not continuous and has an opening referred to as “fill hole”. The panel is then placed in a vacuum chamber to vacate the air from within the panel. After this step, and while still under vacuum, liquid crystal is introduced to the fill hole. The liquid crystal then fills the gap inside the panel due to capillary forces. This may be accelerated by bringing the panel to atmospheric pressure after the liquid crystal introduction to the fill hole. The process is completed once the liquid crystal has filled the panel gap. However, to avoid future problems (e.g. shrinkage, formation of bubbles, etc.) the amount of liquid crystal in the panel is more than the anticipated volume. As such, the panels are then pressed to remove the excess liquid crystal by a process referred to as “cold pressing”. The fill hole is then sealed using a secondary epoxy to avoid air from entering the panel.
This process is difficult to execute in large area panels because the filling time is proportional to the panel area, so the waiting time needed for the filling to complete may take several hours for each panel. This process is uneconomical, especially with the additional time required to vacate the air in the vacuum process. Furthermore, the control of cell gap becomes exceeding difficult.
The use of flexible substrates in the traditional vacuum process poses another difficulty. When air in the chamber is vacated any trapped air in the empty cell causes the empty cell to expand, much like a balloon. This could lead to damage of the cell or breaking of the cell gasket. Extra precautions are needed, such as sandwiching the flexible cell between two ridged materials to prevent ballooning, for the vacuum fill process.
To mitigate this, a new process, referred to as one drop filling (ODF) was invented. In this process, the glass substrate is coated. A gasket is deposited around the entire perimeter of the glass substrate. The substrate is then placed in a large vacuum chamber. A second glass substrate is also placed in the vacuum chamber and is held above the original substrate. At this point, a dispenser deposits the exact amount of liquid crystal that will be needed on the bottom glass substrate. Once vacuum state is achieved, the two substrates are brought together. The epoxy gasket is cured creating a sealed system. The liquid crystal fills this panel through capillary forces. The panel can be brought to atmospheric pressure to accelerate the filling process as before. The advantage of the ODF method is that the cold press step is omitted. Furthermore, the system can reduce the process time, especially for large area panels.
An important aspect of these processing methods is that the final panel is considered to be under negative pressure. In other words, since the panel is fabricated under vacuum, the inside pressure is considered to be lower than the atmospheric pressure. This means that air will penetrate the panel if given the opportunity. Therefore a breach in the gasket will result in catastrophic failure of the panel. To avoid this problem, the gaskets are designed to be impenetrable to air.
Glass based panels cannot be used in applications in which durability, flexibility, or light-weight is of importance. Such applications include eyewear, protective shields, highly curved windows/displays, etc. Therefore, there is a demand for flexible plastic LC devices.
The manufacturing methods used for liquid crystal panel fabrication are not fully compatible with plastic substrates. For one thing, plastic is flexible, making the handling of plastic substrates particularly difficult in fabrication processes. The lack of flexibility of glass which is considered a drawback for many applications is in fact necessary for the fabrication processes stated above. While some small area plastic cells have been made using conventional processes above, the low yields have limited their introduction. This is primarily due to the stringent conditions needed for any vacuum filling process. Furthermore, once the panel is fabricated, the plastic based devices have a significantly lower lifetime. This is due to the fact that plastics are permeable materials, allowing transfer of gasses. Since the panels are fabricated under negative pressure, air will eventually enter the cell. This has significantly limited introduction of plastic based liquid crystal devices. While many companies (e.g. Teijin, DuPont, Mitsubishi etc.) have been working on hard coats to reduce the gas permeability of plastic substrates, they have not yet reached the values offered by even the thinnest glass.
Some liquid crystal devices based on plastic have emerged in the market. They attempt to overcome these issues by processing the system in atmospheric pressure. A method of achieving this is to eliminate the gasket seal and use a roller to place the liquid crystal on the substrates. However, to avoid the liquid crystal from coming out of the panel because of lack of gasket seal, they introduce a significant amount of polymer in the liquid crystal. In this method, the liquid crystal material is “encapsulated,” meaning a quantity of liquid crystal material is confined or contained in an encapsulating medium. Such microencapsulation prevents the liquid crystal from “flowing,” making manufacture of large displays possible. The polymer encapsulated liquid crystal creates micro “panels” within the large panel. The polymer also helps maintain the cell gap by adhering to the two substrates. These materials most commonly known as Polymer Dispersed Liquid Crystal (PDLC), Nematic Curvilinear Aligned Phase (N-CAP), Polymer Stabilized Cholesteric Texture (PSCT), Polymer Encapsulated Liquid Crystal (PELC), and Polymer Network Liquid Crystal (PNLC), etc. have a significant drawback in that they do not exhibit optical clarity and are hazy due to light scattering by the encapsulated liquid crystal domains. This has limited their use to privacy applications (e.g. privacy windows, etc.). It should be noted that these systems lack the stability of the glass panels because of absence of the gasket. In particular, air and moisture penetrates the panel over time and renders the product inoperable. As such, these systems have not achieved marketability. To overcome this limitation, the encapsulation size by the polymer was increased. Furthermore, patterned micropanels were created to limit the flow of the liquid crystal in the final large panel. However, these additional structures reduce the optical performance of the cell and create additional effects such as diffraction. In optical device applications, a device without the presence of these polymer walls and structures are needed to avoid any optical artifacts in the viewing area.
Other proposed solutions include, e.g., US Patent Application 2009/0128771, entitled “Fabrication Methods for Liquid Crystal Display Devices” (Yang et al.), which describes a roll-to-roll method of manufacturing cells using a “patterned enclosure structure” that includes a plurality of stripes for dividing liquid crystals. Another method uses patterned micro-polymer spacers to contain LC material within small confined spaces. For example in a method described in Wen-Tuan Wu et al. “P-55: Cell filling of Flexible Liquid Crystal Displays Using One Line Filling and Tilted Roller Pressing”, SID 07 Digest, p 393 (2007), micro-polymer spacers that are 10 μm wide×170 μm long×3 μm high are formed on one substrate in order to contain the liquid crystal material in small, rectangular spaces, therefore making manufacture of a large cell possible. Other examples of patterned spacers include the method of Liang et al., U.S. Pat. No. 7,850,867 entitled “Compositions for liquid crystal display.”
Other methods include providing a “support layer” made of a material capable of absorbing or binding LC material so as to make the LC layer dimensionally stable in thickness and of sufficient thickness to perform. See U.S. Pat. No. 5,868,892.
While plastic substrates lend themselves to a roll-to-roll type of manufacturing with reduced costs and increased manufacturing efficiency, previous efforts to implement a roll-to-roll continuous manufacturing process for various flexible displays have not been successful. The manufacture of large surface area flexible displays has been particularly illusive. One reason is that in liquid crystal devices, such as displays or optical devices, it is essential that the liquid crystal layer (i.e. the liquid crystal material together with any dyes mixed therein) have an optimum uniform thickness, because variations in thickness cause variations or gradations in optical properties of the liquid crystal device. In addition, the varying thickness of the liquid crystal material will cause corresponding variations in the electrical properties of the liquid crystal material, such as capacitance and impedance, further reducing uniformity of a liquid crystal device, especially one with a large size. The varying electrical properties of the liquid crystal material may also cause a corresponding variation in the effective electric field applied across the liquid crystal material. Additionally, in response to a constant electric field, areas of the liquid crystal that are of different thicknesses would respond differently. Thus, there should also be an optimum spacing of the electrodes by which the electric field is applied to the liquid crystal material. To maintain such optimum thickness and spacing, rather close tolerances must be maintained. To maintain close tolerances, there is a limit as to the size of the device using such liquid crystals, for it is quite difficult to maintain close tolerances over large surface areas. In addition, the amount of liquid crystal must be controlled as is the case in vacuum based processing. However, in a rolled based plastic process, the presence of vacuum is best avoided for the reasons stated above.
For these reasons, large size single cell liquid crystal devices, such as for example a sunroof or window, have not been made satisfactorily, mainly because of the fluidity of the liquid crystals, i.e. the tendency of the material to flow, creating areas that have different material thicknesses resulting in non-uniform optical and electrical characteristics.
Generally, it has been conventional thought that other than using the various encapsulation/patterned spacer methods described here, it is not possible to make a flexible cell filled with a fluid electro-optical mixture, such as a liquid crystal, using a roll-to-roll, roll to sheet, roll to part or continuous manufacturing process (here collectively referred to as roll-to-roll). This is because of the difficulty of working with flexible plastics, having to maintain a controlled distance between the two substrates at about 5-20 μm with only a variation (tolerance) of +/−1-2 μm; the precision required to fill the controlled gap between the top and bottom substrates with an amount of liquid crystal sufficient to fill the entire gap without forming bubbles or defects; and the fluid nature of the liquid crystal, which requires either having to stabilize the LC using polymerization or encapsulation, and/or having to use spacers that can form discrete patterns, all of which result in “haze” which is undesirable.
Therefore, there remains a demand for an efficient manufacturing method for flexible, plastic, substantially polymer-free liquid crystal devices.